The present invention is related to an improved process for forming a capacitor and a capacitor formed thereby. More specifically, the present invention is related to the optimization of a polymer layer in a solid electrolytic capacitor for improved humidity performance.
Solid electrolytic capacitors with conductive polymers as the cathode material have been widely used in the electronics industry due to their advantageously low equivalent series resistance (ESR) and their non-burning/non-ignition failure mode. The typical compositions and properties of polymer slurry are illustrated in U.S. Pat. No. 5,300,575 which is incorporated herein by reference.
U.S. Pat. No. 6,391,379, which is incorporated herein by reference, discloses a process that involves multiple coating-drying process steps, thereby significantly simplifying the process of polymer dispersion.
U.S. Pat. No. 7,563,290, which is incorporated herein by reference, describes the slurry/dispersion process. The resulted capacitors show excellent high voltage performances, reduced DC leakage (DCL) and improved long term reliability.
It is highly desirable that the capacitor devices are of high reliability and that they can withstand stressful environments. Therefore, the integrity of the anodes and the robustness of conductive polymer cathode are essential for high quality capacitor products. However, it is a challenge to form a conductive polymer coating on the anodes that is defect-free, and which is capable of withstanding thermal mechanical stress during anode resin encapsulation and surface-mounting. The improper application of polymer slurry often leads to the formation of cracks and delaminating of the polymer coating thus formed.
In a manufacturing process to produce conductive polymer based valve metal capacitors the valve metal powder, such as tantalum, is mechanically pressed into anodes that are subsequently sintered to form porous bodies. The anodes are anodized to a pre-determined voltage in a liquid electrolyte to form a dielectric layer onto which a cathode layer of conductive polymer is subsequently formed via an in situ polymerization process comprising multi-cycle oxidizer/monomer coatings and polymerization reactions. The anodes are then coated with graphite and Ag followed by assembling and molding into a finished device.
A particular concern is the formation of adequate polymer coatings on edges and corners. In order to achieve good quality polymer coating on anodes, especially on the corners and edges, many types of chemical compounds are used for either forming a pre-coating on the anode or which are added to the polymer slurry. U.S. Pat. No. 7,658,986, which is incorporated herein by reference, describes the difficulty in coating the edges and corners of the anode with polymer slurry. These materials tend to pull away from the corners and edges due to surface energy effects. The resulting thin coverage at corners and edges leads to poor reliability of the device.
In general, the formation of coatings on edges is well understood. In fact, the magnitude of the force pulling the liquid away from the edge is given by the Young and Laplace: Δp=γ/r
wherein:
Δp=the pressure difference causing the liquid or slurry to recede from an edge;
γ=the surface tension of the liquid or slurry; and
r=the radius of curvature of the edge.
This effect of liquid distribution on corners and edges of an anode is illustrated in FIG. 1.
One approach to mitigating poor coverage of the anode corners and edges has been to alter the design of the anode as disclosed in U.S. Pat. Nos. 7,658,986, D616,388, D599,309, and D586,767 each of which is incorporated herein by reference. While changes in the anode design are beneficial in some regards the effect of poor coverage is still present even with anode designs which facilitate corner and edge coverage by polymer slurry as the primary cathode layer.
Another approach for improving coverage of the corners and edges is provided in International Application WO2010089111A1, which is incorporated herein by reference, which describes a group of chemical compounds called crosslinkers, which are mostly multi-cationic salts or amines, such as an exemplary material decanediamine toluenesulfonate. International Application WO2010089111A1 teaches the application of a solution of the crosslinker on the anodes prior to the application of polymer slurry to achieve good polymer coverage on corners and edges of the anodes. The effectiveness of the crosslinker is attributed to the cross-linking ability of multi-cationic salts or amines to the slurry/dispersion particles. While crosslinkers are advantageous for improving the coating coverage on corners and edges of the anodes, the addition of these crosslinkers, which are mostly ionic in nature, has the unintended consequences of degrading the humidity performance of finished product.
Removing the crosslinker has not been considered feasible. It is widely accepted that the polymer layer is not to be washed due to expected disruption in the bonding between the polymer and the underlying surface, furthermore, it is widely known that the polymer layers are susceptible to swelling and dedoping resulting in an increase in ESR. U.S. Pat. No. 6,391,379 for example, which is incorporated herein by reference, teaches redissolution of the polymer layer. U.S. Pat. No. 6,987,663 and U.S. Pat. No. 7,377,947, both of which are incorporated herein by reference, teach that contacting the polymer layer with an aqueous electrolyte may cause the polymer slurry layer to detach from the anode body. Therefore, one of skill in the art would be expected to avoid washing the polymer layer.
There has been a long standing challenge to make a capacitor with low ESR and adequate humidity performance.